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Effect of Tensioning a Boring Bar

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ken king, King Design06/02/2020 23:03:01
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I remember that in late 50's/early 60's at the College of Aeronautics (as was), Cranfield, experiments were being carried out to examine and quantify the increase in strength of hollow struts which could be achieved by pressurising them. Being struts they were by definition loaded in compression, and the pressure would induce lonitudinal tension stress as well as hoop stress. If any of you can make a connection between this and boring bars I'd like to read it,

Cheers, Ken

Michael Gilligan07/02/2020 06:56:46
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Posted by ken king, King Design on 06/02/2020 23:03:01:

I remember that in late 50's/early 60's at the College of Aeronautics (as was), Cranfield, experiments were being carried out to examine and quantify the increase in strength of hollow struts which could be achieved by pressurising them. Being struts they were by definition loaded in compression, and the pressure would induce lonitudinal tension stress as well as hoop stress. If any of you can make a connection between this and boring bars I'd like to read it,

Cheers, Ken

.

My tentative, but hopefully not too tenuous connection is that both examples are conceptually similar to ‘pre-stressed concrete’ beams ... and that there may be ‘accessible’ descriptions available of of those.

I personally don’t have the math to do the calculations for a pre-stressed boring bar, but the concept seems reasonably intuitive .

MichaelG.

.

P.S. We once had a forum member who was willing and able to produce Finite Element models; but he left when the chorus of ‘practical engineers’ decried his contributions.

Neil Lickfold07/02/2020 10:23:23
836 forum posts
166 photos

I can't do the math, but liken it to a plastic bag tube. Put pressure in it, and it is substantially more rigid than when the pressure drops, etc

Sandvik did boring bars with oil and a piston. As it is being used, (in a lathe) you can adjust the pressure and tune it to the situation it is in. I have not made one yet, am thinking about using it to fix a bar and see what happens.

When I get it done, will post the results here. The bar I am going to try has a 12mm diameter shank and a min bore diameter of 14mm. It would be great to be able to do the math t hing and figure out that if it has 120psi it becomes this rigid for example.

Neil

Graham Meek07/02/2020 11:09:16
465 forum posts
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Sandvik is a very good site for an insight into tips on the boring operation. Whilst biased more for industry there are a few gems of information hidden in the text.

There were several things that I picked up on. One the L/D 4 bar protrusion for a standard bar. Another which GHT seems to have got right form the start, is that Sandvik recommend mounting bars in sleeves rather than just clamping them in the toolholder.

Another thing I found interesting and one which I have found from experience. Was the size of the tip radius should not exceed the depth of cut. Lastly the diagram showing the tool forces during boring. These show, IF, the magnitude of the arrows is correct. That more of the cutting force is trying to push the bar out of the cut, ie along the diameter, than is trying to deflect the bar downwards. Obviously the resultant force will be combination of the two, (Vector Analysis).

Regards

Gray,

Henry Brown07/02/2020 11:21:13
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Won't comment on any of the previous but when I worked all boring bars were made in house, they were upto three feet long and a couple of inches AF, sometimes larger. They were always made from nitriding steel and nitrided as it was thought it made them more resistant to chatter.

ega07/02/2020 11:27:56
2500 forum posts
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Posted by Graham Meek on 07/02/2020 11:09:16:

...

There were several things that I picked up on. One the L/D 4 bar protrusion for a standard bar. Another which GHT seems to have got right form the start, is that Sandvik recommend mounting bars in sleeves rather than just clamping them in the toolholder.

I notice from old mart's link that Cutwel do a

Quick Change Anti-Vibration Tool Holder for Imperial Boring Bars

which looks very much like the GHT sleeve but costs over £118!

Thanks for those other useful pointers. I understand that the shape of the cutting edge also affects the direction in which the tool is pushed away from the bore - perhaps, therefore, another reason for using shop-made or modified cutting tools.

Edited By ega on 07/02/2020 11:28:18

Edited By ega on 07/02/2020 11:28:55

Meunier07/02/2020 17:10:49
448 forum posts
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Posted by Neil Lickfold on 07/02/2020 10:23:23:

./.....can't do the math, but liken it to a plastic bag tube. //The bar I am going to try has a 12mm diameter shank and a min bore diameter of 14mm. It would be great to be able to do the math t hing and figure out that if it has 120psi it becomes this rigid for example.

Neil

Neil, I like your plastic bag tube example and confess I am also unable to do the math.
Unless I have completely misunderstood your proposed bar dimensions, the min bore id of 14mm appears to be greater than the 12mm shank diameter.
DaveD.

Edited By Meunier on 07/02/2020 17:11:25

Martin Connelly07/02/2020 17:41:01
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DaveD, diameter being bored not the internal boring bar diameter.

Martin C

duncan webster07/02/2020 18:29:37
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The pressurised bag and prestressed concrete analogies are not all that relevant. Concrete has a very low tensile strength, and so it is sometimes prestressed into compression so that when a bending moment is applied, say by end loading a cantilever, the concrete in the side which would have gone into tension actually just reduces its compression. (Experts please excuse my simplificaton!) Similarly the wall of a plastic bag cannot normally withstand compression, but inflating it puts the wall in tension so it can take a small bending load without the compression side buckling. Steel has the same young's modulus in tension and compression, it is governed by inter-atomic forces and as Graham says is little changed by hardening. It is mainly down to chemistry, and non stainless steels are mainly iron, even alloy steel like en24 is >95% iron

The downside of prestressed concrete comes when you try to demolish it, unless you are very careful, releasing the pre-stress can result in a sudden release of strain energy throwing bits of concrete all over the place. Think wire rope snapping.

I've no doubt Graham is right but the reason must lie elsewhere

Michael Gilligan07/02/2020 18:34:55
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Posted by duncan webster on 07/02/2020 18:29:37:

The pressurised bag and prestressed concrete analogies are not all that relevant. Concrete has a very low tensile strength, and so it is sometimes prestressed into compression so that when a bending moment is applied, say by end loading a cantilever, the concrete in the side which would have gone into tension actually just reduces its compression. […]

.


I beg to differ, Duncan

... I only said that the cases were ‘conceptually similar’

Changing the sign of the ‘stressing’ from positive to negative seems fair to me for comparisons at this level.

MichaelG.

duncan webster07/02/2020 20:05:29
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Well we'll just have to agree to differ then

Michael Gilligan08/02/2020 00:23:05
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Interesting, and fairly recent article here: **LINK**

https://res.mdpi.com/d_attachment/applsci/applsci-09-03541/article_deploy/applsci-09-03541.pdf

MichaelG.

Michael Gilligan08/02/2020 00:34:16
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Posted by duncan webster on 07/02/2020 18:29:37:

.

[…] the concrete in the side which would have gone into tension actually just reduces its compression. […]

Steel has the same young's modulus in tension and compression,

.

... and yet we happily use pre-loaded springs on vehicles, etc. ... Mmm

MichaelG.

duncan webster08/02/2020 16:55:20
3945 forum posts
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Posted by Michael Gilligan on 08/02/2020 00:34:16:
Posted by duncan webster on 07/02/2020 18:29:37:

.

[…] the concrete in the side which would have gone into tension actually just reduces its compression. […]

Steel has the same young's modulus in tension and compression,

.

... and yet we happily use pre-loaded springs on vehicles, etc. ... Mmm

MichaelG.

So your point is ????????

Michael Gilligan08/02/2020 17:31:30
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It’s O.K. Duncan ... We have agreed to differ

I was just musing to myself.

MichaelG.

Meunier08/02/2020 19:35:18
448 forum posts
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Posted by Martin Connelly on 07/02/2020 17:41:01:

DaveD, diameter being bored not the internal boring bar diameter.

Martin C

Martin, thank you for pointing out what is now the obvious. idea
DaveD

DrDave08/02/2020 21:11:11
240 forum posts
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My understanding of this is that the two parts of the boring bar, the tube and the rod up the centre, have different natural frequencies. Then, if one is excited near its resonant frequency and wants to “sing” the other is far from its resonant frequency and is “dead”. One part will thus act to reduce the resonant peak of its neighbour, effectively damping the system.

Michael Gilligan09/02/2020 06:20:14
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This is about large structural steel beams, prestressed by tensioned cables: **LINK**

https://www.researchgate.net/profile/Ahmer_Wadee/publication/265338326_Tensile_performance_of_prestressed_steel_elements/links/5a1323af0f7e9b1e572d74fb/Tensile-performance-of-prestressed-steel-elements.pdf?origin=publication_detail

So ... it might take some mental juggling to see the relevance to boring bars prestressed by push rods ... but it’s an impressive and convincing piece of work.

MichaelG.

Graham Meek09/02/2020 11:09:36
465 forum posts
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Having been reading extensively some of the documents posted, and others. I have come to the following conclusion.

Whether that conclusion is right or wrong, I cannot say, but it holds water for my way of thinking.

The boring bar is a cantilever beam with a load on the extreme free end. There is also a torsional element that is trying to twist the cutting tool about the centre-line. This twisting element will be greater the further the tool tip is away from the centre-line

Looking end on and using the points of the compass, with the tool bit in the East. As the tool starts to cut there will be a bending moment about the N-S axis pushing the tool away from the operator. There will also be a bending moment about E-W pushing the tool downwards. The resultant effect of these two loads will be to twist the cutting tip about the centreline.

In both cases the bending moments are resisted by tension in the North side and East side of the bar. When this tensile stress is within the strength of the bar everything is dandy.

However when the the tensile stresses exceed what he bar can handle then the twist in the bar becomes more prominent. The tool bit swings out of the cut, and as soon as there is no load on the bar immediately returns, or tries to return to the cut. The result of this is chatter or vibration.

If the induced, or static tension, (pre-tension), in the bar exceeds the last condition, then the bar continues to cut as it would in the case prior to chatter.

Whether there is a third element that alters the natural frequency of the bar or not, I have not been able to come to any conclusion on.

Regards

Gray,

Mick B109/02/2020 11:53:15
2162 forum posts
119 photos
Posted by SillyOldDuffer on 17/08/2017 17:29:14:

...

Push rod hand tight - 2.25 seconds

Push rod plus 1/2 turn - 1.41 seconds

Push rod plus 1 turn - 1.35 seconds

Push rod plus 1½ turns - 1.31 seconds

Note the improvement isn't linear with increasing tension.

...

I'm struggling to understand :

i) how you can judge the cessation of visible vibes within a few hundredths of a second with a stoppwatch, or

ii) that nobody else seems to've asked the same question

Did I get bored and start to skip-read? Sometimes it's hard to tell... winkdevillaugh

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